Key fobs used with vehicle passive entry systems are able to measure the received signal strength from a vehicle-originating signal, in order to feedback the signal strength information to the vehicle, so that the location of the key fob with respect to the vehicle can be calculated. The location calculation accuracy depends upon the accuracy of the entire system from a low frequency (“LF”) transmission from the vehicle to the measurement of that power received at the key fob. The vehicle-side transmission can be rather accurately determined, because a closed-looped measurement of antenna power can be made based on current and voltage measurements within a drive circuit associated with the vehicle-side transmitting antenna.
The sensitivity of a receiving circuit (including antennas and other circuits) on the fob can be determined by a measurement made in a controlled environment at the fob manufacturing plant. The antenna on the fob measures the vehicle-originating LF transmission in three mutually perpendicular axes (x-axis, y-axis and z-axis). Three sensitivity measurements are taken, one for each axis. If the sensitivity measurements result in a value that is outside a predefined tolerance, then the fob is discarded. Because of this predefined tolerance, the vehicle-side receiving antenna knows the sensitivity “window,” which is based on the predefined tolerance, every fob falls into, so that a reasonable calculation can be made for the position of the fob with respect to the vehicle-side transmitter.
This system allows for fobs to be easily added to the system as a service part, with no concern for learning the new fob's sensitivity. There are problems, however, with this current system. The accuracy of the location of the fob is limited because the “window” for allowable tolerance must remain somewhat wide, for mass production variation of the design and its components. Narrowing the tolerance of the “window” would result in an increase in the location accuracy, but would also increase the number of fobs that must be discarded for falling outside the predefined tolerance. One manner to overcome this problem of too many fobs falling outside the predefined tolerance would be to sort the fobs into different categories having different tolerance “windows.” From a manufacturing standpoint, sorting is never preferred because scrap is generated and this cost is transferred to the vehicle manufacturer and then to the end customer.